Radio frequency transceivers in cellular systems commonly receive and decode a desired signal in the presence of interference, which has commonly required a compromise in receiver performance. For example, in order to prevent clipping due to interference, several stages of narrow analog filters are typically found in conventional receiver designs. Such filters add current drain and distort the desired signal, thus degrading receiver performance. Additionally, the active stages of the receiver, particularly the radio frequency (RF) stages, are designed with high levels of linearity so that distortion is minimized in the presence of interference. This linearity often requires relatively high bias conditions and therefore requires relatively high current drain.
A typical prior art receiver architecture is shown in FIG. 1. This architecture represents a typical receiver implementation and is described in U.S. Pat. No. 6,498,926 to Ciccarelli et al. Within receiver 100, the transmitted RF signal is received by antenna 112, routed through duplexer 114, and provided to low noise amplifier (LNA) 116, which amplifies the RF signal and provides the signal to bandpass filter 118. Bandpass filter 118 filters the signal to remove some of the spurious signals which can cause intermodulation products in the subsequent stages. The filtered signal is provided to mixer 120, which downconverts the signal to an intermediate frequency (IF) with a sinusoidal signal from local oscillator 122. The IF signal is provided to bandpass filter 124, which filters spurious signals and downconversion products prior to the subsequent downconversion stage. The filtered IF signal is provided to variable gain amplifier (VGA) 126, which amplifies the signal with a variable gain to provide an IF signal at the required amplitude. The gain is controlled by a control signal from AGC control circuit 128. The IF signal is provided to demodulator 130, which demodulates the signal in accordance with the modulation format used at the transmitter (not shown).
For this prior art architecture, the local oscillator signal (LO) is either tuned to match the radio frequency signal (RF), so that the received signal is converted directly to baseband, or it is tuned to convert the received RF signal to some much lower intermediate frequency (IF) for further filtering. At baseband or IF, the filters are set to the bandwidth of the particular RF system to receive the desired signal and remove interference.
The architecture in FIG. 1 is designed to receive the desired signal in the presence of interference. The filter at baseband or IF is set to remove completely any interference, and the RF stage gain and bias are set to receive the signal with interference with minimal distortion. Thus, such a conventional system makes assumptions about the presence of interference, which may reduce interference at the expense of receiver performance when the expected interference is present, but which may constitute a wasteful approach when such assumptions are incorrect.
Another prior art receiver architecture is disclosed at FIG. 2 of U.S. Pat. No. 6,498,926 to Ciccarelli et al. In this prior art architecture, post-demodulation quality is used to set the bias conditions and therefore the linearity of the RF circuits. This prior art approach does not address the problem fully because the receiver state is adjusted based only on the baseband data quality measurement, which might be degraded for numerous reasons and not just due to interference and/or reduced RF linearity. Also, this architecture does not do anything to reduce the filtering requirement to match the actual interference conditions.
Another prior art receiver architecture is disclosed at U.S. Pat. No. 6,670,901 to Brueske et al. This prior art architecture includes an on-channel power detector, a wide band power detector, and an off-channel power detector. The wideband detector and off-channel detector will indicate if high levels of interference are present and allow adjustment of the receiver bias based on that. This prior art architecture suggests using the information from these power detectors to adjust the dynamic range of several blocks (LNA, mixer, filter, analog-to-digital (A/D) converter, and digital filter). By adjusting the dynamic range and/or bias of these stages, the current drain can be optimized. However, this prior art approach uses wideband detection without selectivity and therefore is unable to distinguish out-of-band interference, i.e., interference that is several channels away, from nearby interference in the adjacent or nearby channels. Therefore, the architecture cannot fully optimize the performance of the receiver.
Since an actual device such as a cellular phone operates in a dynamic and changing environment where interference is variable, it is desirable to be able to modify the receiver's operational state depending on the interference.